1. Field of Invention
The present invention relates to a driving method for an organic electro-luminescence display device, a driving method for an electro-optical device suitable for use with an organic electro-luminescence display device the electro-optical device, and an electronic apparatus provided with such an electro-optical device.
2. Description of Related Art
Attention is being given to organic electro-luminescence (electric-field luminescence) display devices using organic materials as luminescent materials of luminescent elements since they have a wide viewing angle, and will potentially meet market demands, i.e., demands for thinner, lighter, smaller, and lower power-consuming display devices.
Unlike conventional liquid crystal display devices, in organic electro-luminescence display devices, the luminescence state of the luminescent element must be controlled by a current. One such control method is the conductance control method (T. Shimoda, M. Kimura, et al., Proc. Asia Display 98, 217; M. Kimura, et al., IEEE Trans. Ele. Dev. 46, 2282 (1999); M. Kimura, et al., Proc. IDW 99, 171; and M. Kimura et al., Dig. AM-LCD 2000, to be published). In this method, the luminescence state of the luminescent element is controlled by a current value, which is an analog value, and more specifically, it is controlled by changing the potential applied to a gate electrode of a driving transistor for driving the luminescent element. When thin-film transistors having different current characteristics are used, however, the difference in the current characteristics of the individual transistors may sometimes directly result in non-uniformity in the luminescence state of the luminescent elements.
Accordingly, the area ratio gray-scale method (M. Kimura, et al., Proc. Euro Display '99 Late-News Papers, 71, Japanese Unexamined Patent Application Publication No. 11-073158, M. Kimura, et al., Proc. IDW 99, 171, M. Kimura, et al, J. SID, to be published; and M. Kimura, et al., Dig. AM-LCD 2000 to be published) has been proposed. In the area ratio gray-scale method, unlike the above-mentioned conductance control method, the luminescence state of the luminescent elements is controlled without using a luminescence state at an intermediate luminance. More specifically, in this method, a gray-scale is displayed as follows. Pixels disposed in a matrix are divided into a plurality of sub-pixels, and it is determined whether the luminescent elements contained in the sub-pixels are either in a complete luminescence state or a complete non-luminescence state. Then, among the plurality of sub-pixels, the total area of the sub-pixels in the complete luminescence state is changed. In the area ratio gray-scale method, it is not necessary to set an intermediate current value corresponding to the luminescence state of the intermediate luminance. Accordingly, the influence of the current characteristics of the transistors for driving the luminescent elements can be reduced, thereby achieving a uniform image quality. In this method, however, the number of gray-scale levels is restricted by the number of sub-pixels. For a greater number of gray-scale levels, pixels must be divided into a greater number of sub-pixels, which makes the pixel structure complicated.
Accordingly, the time ratio gray-scale method (M. Kimura, et al., Proc. IDW 99, 171; M. Kimura, et al., Dig. AM-LCD 2000, to be published; M. Mizukami, et al., Dig. SID 2000, 912; and K. Inukai, et al., Dig. SID 2000, 924) has been proposed.
In the time ratio gray-scale method, the representation of the gray-scale is implemented by changing the period for which the luminescent elements in one frame are in the complete luminescence state. Accordingly, unlike the area ratio gray-scale method, it is not necessary to provide many sub-pixels to obtain a greater number of gray-scale levels. Also, the time ratio gray-scale method can be used together with the area ratio gray-scale method. Thus, it is expected that the time ratio gray-scale method will be a promising method for digitally displaying a gray-scale.